To construct load-bearing asphalt pavements, hot mix bituminous paving compositions are most commonly used. Hot mix bituminous paving compositions consist principally of aggregate and bitumen binder. They are generally produced by mixing preheated aggregate and hot bitumen binder in either batch or continuous mixing equipment. Prior to mixing with the hot bitumen binder, the aggregate is heated to temperatures exceeding 150° C. to quantitatively remove both surface and pore-bound moisture. The bitumen binder is heated to temperatures typically exceeding 143° C. to lower the viscosity of the product and make it suitable for pumping through conventional liquid transfer equipment such as pumps. The resulting hot mix bituminous paving composition typically has a temperature exceeding 143° C. upon exiting the mixing equipment. This high-temperature bituminous paving composition is typically referred to by those skilled in the art as hot-mix asphalt (“HMA”). Elevated temperatures are needed in the production of HMA paving composition for two main reasons. First, high temperatures ensure complete removal of water from the aggregate. Wet aggregate cannot be coated by bitumen binder. Second, high temperatures ensure that the bitumen binder remains low in viscosity. Low bitumen binder viscosity enables the finished HMA to be workable throughout all the unit operations of mix production and pavement construction.
It is also known that oil-in-water bitumen emulsions and water-in-oil bitumen dispersions (inverted emulsions) may be used to produce bituminous paving compositions and construct load-bearing bituminous pavements at temperatures lower than those typical of the HMA process.
In U.S. Patent Publication Nos. 2007/039,520 and 2008/060,551, Crews et al. teach the production of bituminous paving compositions suitable for asphalt pavement construction through a process involving the mixing of a bitumen emulsion with aggregate at temperatures ranging from 60° C. to 140° C., wherein the oil-in-water bitumen emulsion comprises about 50-75% by weight of bitumen, about 0.05%-2.0% by weight of emulsifier, and about 25%-50% by weight of water. The bitumen emulsion is delivered to the worksite and mixed with aggregate in a high-temperature mixing equipment to produce bituminous paving composition. As such, the levels of water added to the high-temperature mixing equipment would exceed 1.0% by weight of the bituminous paving composition. This water is converted to steam in the mixing equipment, and this steam may cause fouling in the mixing process equipment such as filters and bag houses. Additionally, the steam may mix with airborne dust (produced during the mixing process) to form a mud-like barrier within the air filters of the plant exhaust equipment. The mud-like barrier restricts air flow through the exhaust equipment, thus causing a reduction of the mix production rate.
In U.S. Pat. No. 7,297,204, Crews et al. teach the production of bituminous paving compositions suitable for asphalt pavement construction through a process involving the mixing of an inverted bitumen emulsion with aggregate at temperatures ranging from 60° C. to 140° C., wherein the inverted bitumen emulsion comprises about 75-95% by weight of bitumen, about 0.05%-2.0% by weight of emulsifier, and about 5%-25% by weight of water. The inverted bitumen emulsion is delivered to the worksite and mixed with aggregate in a high-temperature mixing equipment to produce bituminous paving composition. The water in the bitumen inverted emulsion is converted to steam in the mixing equipment, which could foul the mixing process equipment and form a mud-like barrier that restricts air flow through the exhaust equipment.
Accordingly, there is a need for a process of producing bituminous paving compositions that are suitable for construction of load-bearing bituminous pavements at temperatures lower than those typical of the HMA process, wherein the production process minimizes the aforementioned problems caused by the water steam generated in the mixing operation upon mixing of aggregate with bitumen binder.